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1 Chapter 4 Network Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR Edited by Lindsay, Marga, and Sarah - March 2003 All material copyright 1996-2002 J.F Kurose and K.W. Ross, All Rights Reserved

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4 RIP ( Routing Information Protocol) Distance vector algorithm Distance/Cost metric: Each link has a cost of 1. Maximum path cost is 15 – limits use of RIP to AS’s with a diameter of fewer than 15 hops. Distance vectors: exchanged among neighbors every 30 sec via RIP Response Messages (also called advertisements) Each advertisement: list of up to 25 destination routers and their distances from the router Each forwarding table has at least one row for forwarding to networks outside the AS

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7 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops Routers can request info from neighbors about cost to a given destination using a request message Request and response messages are sent over UDP and UDP Packet is carried in a standard IP packet

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12 Hierarchical OSPF Two-level hierarchy: local area, backbone. Link-state advertisements only in area each node has detailed area topology Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone. Route between areas in the AS Boundary routers: in backbone all outgoing packets are routed to boundary router if going to another AS

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17 BGP: controlling who routes to you X,W,Y are stub networks (all traffic entering them must be destined for them and all traffic exiting them must have originated there.) X is dual-homed: attached to two networks X does not want to route from B via X to C.. so X will not advertise to B a route to C

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18 BGP: controlling who routes to you A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers B wants to force C to route to w via A B wants to route only to/from its customers!

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19 BGP operation Q: What does a BGP router do? Receiving and filtering route advertisements from directly attached neighbor(s). Route selection. To route to destination X, which path (of several advertised) will be taken? Sending route advertisements to neighbors.

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26 Input Port Functions A copy of the forwarding table is stored at each input port and updated as needed The switching decision can be made locally at each input port Decentralized switching avoids a forwarding bottleneck at a single point within the router Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Also known as Decentralized Switching

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27 Complicating Factors Backbone routers must operate at high speeds, so they therefore must be capable of performing millions of lookups per second. Line speed: a lookup is performed in less than the amount of time needed to receive a packet at the input port. Example: Consider an OC48 link that runs at 2.5 Gbps. Assuming a packet size of 256 bytes, this implies a lookup speed of approximately a million lookups per second performed.

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31 Switching Via An Interconnection Network Overcomes bus bandwidth limitations Some interconnection networks were initially developed to connect processors in a single multiprocessor Advanced design: fragments datagram into fixed length cells, then switches cells through the fabric. Cisco 12000: switches Gbps through the interconnection network

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32 Output Ports Transmits the datagrams that have been stored in the output port’s memory and transports them over the outgoing link Buffering is required when datagrams arrive from fabric faster than the transmission rate of the output port Scheduling discipline is used to choose among queued datagrams for transmission onto network

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33 Queuing at the Output Port Buffering occurs when arrival rate via the switching fabric exceeds output line speed Consequently, a delay due to queuing occurs and there is potential packet loss due to output port buffer overflow

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36 IPv6 The 32-bit address space of IPv4 has begun to cause concern. Why? Initial Motivation for creating IPv6 32-bit address space means all possible addresses will be completely allocated by sometime between 2008 and 2018. Although there is a lot of time left until the current address space is exhausted, it will take considerable time to deploy a new technology on such an extensive scale so it is important to start now. IPv6 will have 128 bits for the IP address. This is enough to allow every grain of sand its own IP address!

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39 IPv6 Header A closer look at some of the fields: Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data Traffic Class: Similar idea to the type of service field in IPv4 Checksum: Does not exist in IPv6! It was removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by “Next Header” field

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40 Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneously no “flag days” How will the network operate with mixed IPv4 and IPv6 routers? Two proposed approaches: Dual Stack: some routers with dual stack (v6, v4) can “translate” between formats Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

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41 Dual Stack Approach A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: ?? Src: A Dest: F data Src:A Dest: F data A-to-B: IPv6 Src:A Dest: F data B-to-C: IPv4 D-to-E: IPv4 E-to-F: IPv6 IPv6 nodes have full IPv4 capabilities as well. When operating with an IPv4 node, the IPv6 node uses v4 datagrams. The node will be able to determine the capabilities of the node it is communicating with by looking at the address returned by the DNS.